Electroplating is a well known process for applying metal coatings to an electrically conductive substrate. The process employs a bath filled with a metal salt containing electrolyte, at least one metal anode and a source of direct electrical current such as a rectifier. A workpiece to be plated acts as a cathode.
Nickel electroplating involves the deposition of nickel on a part, immersed into an electrolyte solution and used as a cathode, while the nickel anode is being dissolved into the electrolyte in the form of the nickel ions, traveling through the solution and depositing on the cathode surface.
Common nickel plating baths including bright nickel plating baths, semi-bright nickel plating baths, among others. Bright nickel plating baths are used to provide a decorative appearance on a substrate because of their ability to cover imperfections in the base metal (i.e., leveling). Bright nickel plating baths are used in the automotive, electrical, appliance, hardware and other industries where a bright surface is desired. Semi-bright nickel plating baths are used for engineering purposes where brightness is not desired and were developed in part for their ease in polishing.
The most common nickel plating bath is known as a Watts bath and typically contains about 20-40 oz/gal nickel sulfate, 4-12 oz/gal nickel chloride and 4-6 oz/gal boric acid. The Watts bath is typically operated within a pH range of about 2-5 and at a current density of 20-100 asf. Other plating baths include high chloride solutions, all-chloride solutions, fluoroborate solutions and sulfamate solutions, by way of example and not limitation.
Nickel sulfamate plating baths are based on the nickel salt of sulfamic acid and the pH of the bath is adjusted using sulfamic acid, nickel oxide or nickel carbonate. Nickel coatings from this type of bath typically exhibit very low stress values and high elongations. One advantage of this bath is that it can be operated at higher nickel concentrations (e.g., about 180-200 g/l) which allows for the use of high current densities without losing the properties of the coating. Nickel sulfamate baths typically comprise about 40-60 oz/gal nickel sulfamate, 0-4 oz/gal nickel chloride and 4-6 oz/gal boric acid and are operated within a pH range of 3.5-4.5 and a current density of about 5-260 asf. High nickel concentrations of sulfamate electrolytes permit the conduct electroplating at high current densities (high rates of deposition).
Notwithstanding the type of nickel plating bath that is used, it is often necessary to make chemical additions to the nickel plating bath to increase pH and replenish nickel concentration in the bath.
As discussed above, bright and semi-bright nickel plating baths are typically operated at a pH of between 3.5-4.5. The pH typically rises slowly during operation, since the cathode efficiency is slightly lower than the anode efficiency. Nickel carbonate is a preferred pH adjuster because it dissolves easily at a pH below 4.0. In addition, the temperature range of the plating bath is important in terms of physical properties and, along with agitation, aids in keeping the bath components mixed and solubilized. If the temperature is too high, the addition agent consumption is increased, adding to the expense of operating and plating problems. If the temperature is too low, boric acid in the bath may begin to precipitate and the brighteners will not respond efficiently.
In a typical plating operation, a series of metal anodes are hung from one or more anode bus bars while workpieces to be plated are immersed in the plating bath and attached to a cathode bus bar. The negative terminal of a DC power supply is connected to the cathode bus bar while the positive terminal of the power supply is connected to the anode bus bar. The voltage is adjusted at the power supply to provide a current density on the cathodic workpieces which is considered optimal.
Most nickel plating processes are operated with soluble nickel anode materials. Nickel from the anode is converted into ions which enter the plating solution to replace those discharged at the cathode. In addition, the anode also distributes current to the workpieces to be plated and influences metal distribution. Insoluble anodes, also referred to as inert anodes, do not dissolve during electrolysis because insoluble anodes are comprised of inert material. Typical insoluble anodes include platinized titanium, platinized tantalum platinized niobium, titanium, niobium, stainless steel and other inert materials.
As discussed above, one of the simplest ways to satisfy anode requirements is to suspend nickel bars from hooks placed on an anode bar so that the nickel is immersed in the plating solution. While bars or electrolytic strip may be used as the anode, anode baskets, such as titanium anode baskets, may also be used. The titanium baskets are typically made of titanium mesh strengthened by solid strips of titanium. The mesh facilitates the free flowing of nickel plating solution.
Inert anode plating processes require replenishment of cations in the electrolyte. Thus, the use of inert anodes in electroplated nickel causes the pH of the bath to decrease and the nickel metal concentration to decrease. In response, nickel carbonate and/or lithium carbonate are added to the plating bath to increase the pH. However, these chemicals are expensive and can also be difficult to dissolve. Nickel sulfate and/or nickel chloride may be added to replenish nickel metal in the plating bath. However, the pH adjusting chemicals can be more expensive than nickel metal.
Therefore, it would be desirable to provide a means for increasing pH of the nickel plating bath and replenishing nickel metal in the plating bath that overcomes some of the deficiencies of the prior art.